Win32/mebroot Trojan is a nasty Rootkit, that embeds itself on the MBR (Master Boot Record) of your machine, which initiates itself before any Virus protection can enable. Pretty much all virus protection suites will recognise it, but will not be able to remove it.

Below is a method of removal, which will also fix the damage that has potentially been caused by this trojan:-

Download the latest ComboFix from http://www.bleepingcomputer.com/, let that install the Windows Recovery Console, that you will need to re-write the MBR of your boot disk